Image texture to SSS radius?
It's a vector because you can control the amount of scattering for R, G, and B separately if you wish.  It's a bit weird because there is also a "Subsurface Color", but if you set Subsurface color to white, you can see the effect of changing the Subsurface Radius vector:



if you don't want different behavior per RGB color, just pass the color of your image in for that vector.  Assuming it's a grayscale image the RGB values will be the same. 

Note that it's not really a radius per se; you can't set it to ".5 inches" for example. It's more of a relative amount of scattering per color, and multiplied by the Subsurface value.